1. Field of the Invention
The present invention relates to method, apparatus, and system to build a low-profile, large screen digital rear projection display device.
2. Prior Art
The advent of digital high definition (HD) video technology is causing a phenomenal demand for HD televisions (HDTV) and HD display devices with large screen sizes (greater than 50″ screen diagonal). Several display technologies are poised to address this demand; including Plasma Display Panel (PDP), Liquid Crystal Display (LCD), and micro-display based Rear Projection (RP) display devices that use a digital micro-mirror device (DMD) or a liquid crystal on silicon (LCOS) device. Each of these competing technologies has pros and cons: (1) for a given screen diagonal, PDP and LCD displays are generally more expensive than Rear Projection displays; (2) PDP and LCD displays generally are much thinner than Rear Projection displays; (3) although PDP and LCD displays are generally thin enough to mount on a wall, such displays tend to be heavy enough that mounting on a wall can be difficult; (4) Rear Projection displays are typically more cost-effective than PDP and LCD displays; and (5) Rear Projection displays do not offer a low-profile form-factor, as a result may occupy too much space in a room, often making a Rear Projection display not being the display of choice as a large screen display.
For the purpose of this background discussion, the following parameters are used for quantitative comparison of the form-factor of the various display technologies:                R1 being the depth to screen diagonal ratio; and        R2 being total height to screen height ratio.        
PDP and LCD displays have form factor ratios R1 in the range of 0.12 to 0.15 and R2 in the range of 1.05 to 1.07. In comparison, a typical rear projection display has form factor ratios R1 in the range of 0.4 to 0.5 and R2 in the range of 1.3 to 1.5. In the following background discussion, a display device is viewed to have a low profile when its form factor ratio R1 is in the range of 0.15 or less and R2 ratio is in the range of 1.07 or less.
FIG. 1 illustrates a typical rear projection display device prior art. In general, the rear projection display device 100 is comprised of a projection optical engine 110, folding mirrors 120 and 125, and a screen 130. The projection optical engine 110 is further comprised of projection optics 111, a color wheel 112, a light source lamp 113, a cooling fan 114, a micro-display device 115 (such as a DMD or an LCOS device), with a cooling fin 118, mounted together with the micro-display device drive electronics on the formatter board 117, which in turn is mounted to the chassis of the optical engine 110. The projection optical engine 110 generates the image to be displayed, which is then directed toward the screen 130 by the folding mirrors 120 and 125. The optical engine generates the image to be projected by modulating the light generated by the light source lamp 113, using a micro-display device 115, with the pixel gray scale input after the light passes through the red, green and blue segments of the color wheel 112. The electronics on the formatter board 117 synchronize the operation of the micro-display device with the sequential order of the red (R), green (G) and blue (B) segments of the color wheel 112. The depth of the rear projection display device 100 depends on its projection screen 130 diagonal, the throw ratio characteristics (defined as the ratio of the throw distance to the screen diagonal) of the projection optics 111, and the number folding mirrors 120 and 125 used. Currently marketed rear projection devices with 50″ screen diagonal that use a single folding mirror and an optical engine having 0.45 throw ratio have 15″ depth and 7.5″ of height under the screen, making its form factor ratios R1=0.3 and R2=1.37.
The efficiency of currently marketed rear projection display devices, in terms of luminance output at the screen relative to the luminance input of the light source lamp 113, is typically in the range of 9% to 12%. This poor luminance efficiency of rear projection display devices is primarily attributed to the fact that the color wheel 112 blocks at least ⅔ of the luminance generated by the light source. When combined with the efficiency of the light source lamp 113 itself, the poor luminance efficiency of rear projection display devices contribute to creating a thermal management problem that typically requires at least one cooling fan 114 and an additional cooling fin 11. This thermal management problem tends to put a limit on the luminance of the light source lamp 113 that can be used, thus dictating the need to use a projection screen 130 with a diffusion component as well as a fresnel component to collimate the projection output. The addition of a fresnel component to the screen 130 adds cost and makes the brightness of the screen vary with the viewing angle. In addition to having a typically poor efficiency, the high-pressure arc lamps typically used as a light source lamp 113 needed to generate sufficient luminance for large screen projection also have very poor lifetime and reliability. Such poor reliability when combined with the inherently poor reliability of the motor driven color wheel and cooling fan makes the overall reliability of large screen rear projection devices of FIG. 1 even worse.
Thin rear projection display devices with large screen diagonal have been developed that have a depth in the range of 12″. However, such thinner rear projection display devices typically rely on the use of aspherical mirrors, which are difficult to manufacture and difficult to align, which results in the display becoming expensive (see U.S. Pat. Nos. 6,485,145, 6,457,834, 6,715,886 and 6,751,019). FIG. 2 illustrates another prior art thin rear projection display device 200 that overcomes the use of aspherical mirrors (see U.S. Pat. Nos. 6,728,032 and 6,804,055 and U.S. Patent Application Publication No. 2004/0032653). The thin rear projection display device 200, illustrated in FIG. 2, includes a wide-angle optical engine 210 for projecting an image, a plurality of folding mirrors 220-230, and a screen 240 designed to act as a folding reflector as well as to display the projected image. The screen 240 Fresnel includes a plurality of angular offset reflective elements 250 configured to reflect light incident on the screen from one angle, and to transmit light incident on the screen from a second angle. The thin rear projection display device 200 achieves its thin depth by relying on the use of wide-angle optical engine, with typical throw ratio in the range of 0.12, and multiple folding of the projection light cone. The former technique tends to make the projection optics quite expensive while the latter technique tends to substantially increase the height needed under the screen area. Although the thin rear projection display device 200 of FIG. 2 could achieve a depth to screen diagonal ratio R1 in the range of 0.15, its total height to screen height ratio R2 could be in the range of 1.57. In effect the depth of the thin rear projection display device 200 of FIG. 2 is reduced at the expenses of substantially increasing the rear projection display cost and its total height to screen height ratio R2. Furthermore, the multiple light cone folding employed tends to severely complicate the light cone alignment, making the rear projection display device difficult to manufacture, thus more costly. In addition, the use of exotic fresnel designs combined with the use of multiple light cone folding tend to further degrade the luminance efficiency of the display device, thus making it have poor brightness performance. The use of a wide angle optical engine, a dual purpose screen, with a complex fresnel design, and multiple light cone folding makes the thin rear projection display device 200 of FIG. 2 substantially more expensive than the rear projection display device 100 illustrated in FIG. 1. Further, the thin rear projection display device 200 of FIG. 2 suffers from the same poor reliability performance as that of the rear projection display device 100 illustrated in FIG. 1.
FIG. 3 illustrates a prior art projection display device 300 approach that offers improved reliability (see U.S. Pat. No. 6,224,216). The prior art projection display device 300 attains improved reliability by using light emitting diode (LED) devices as a light source instead of the high-pressure arc lamps typically used as a light source in projection devices. The prior art projection display device 300 includes an array of red (R), green (G) and blue (B) LED devices 310 powered by a power supply 320, a fan 330 for cooling the LED array, an array cover plate 315, an optical fiber bundle 340 that guides the RGB light generated by the LED array to an optical integrator 350, a optical path lens group 360 that direct the generated light toward a micro-display device 370, a projection optics group 380 that projects the image generated by the micro-display device 370, and a display controller 390. The display controller 390 receives color image data from an external source and processes the image data into frame sequential red, green and blue image data, sequential frames of which are conveyed to the micro-display device 370 in proper synchronism with signals sent to the power supply 320 to turn on the array of LED devices 310 that emits the corresponding color. In the prior art projection display device 300 the high-pressure arc lamp and the color wheel, typically used in projection display devices such as display devices 100 and 200, are replaced by the LED array 310 with its emitted color being sequenced by the display controller 390. Although the primary motive of doing so would be to get rid of the inefficiencies, thermal management and reliability problems associated with the high-pressure arc lamp and the color wheel, in the prior art projection display device 300 the LED devices are placed in a relatively close vicinity, thus causing thermal management to reemerge as a problem that requires careful consideration, especially when LED devices performance degrades considerably when their junction temperature rises excessively. Furthermore, the prior art projection display device 300 attempts to use the LED array 310 to literally replace the high-pressure arc lamp as a light source in terms of its aperture and luminance output by using a complex system comprised of the LED array 310, array cover plate 315, optical fiber bundle 340, optical integrator 350, and optical path lens group 360, making the light source assembly quite cumbersome and complex to integrate, thus resulting in a complex and costly display device. In addition, the prior art projection display device 300 does not include any provisions to compensate for the fact that LED devices are nearly impossible to maintain a fixed color-point, because the performance of red, green and blue LED devices degrades at different rates and their color shifts with age and as the temperature changes. Furthermore, the prior art projection display device 300 does not include any provisions for sensing and controlling the color and brightness output of the LED devices, which is critical for maintaining a fixed color-point projection output. Because of the aforementioned weaknesses, the approach for using LED devices as a light source used in the prior art projection display device 300 would not be viable for use in large screen size rear projection display devices.
As it will become apparent in the following detailed description, the large screen size and low-profile rear projection display of this invention utilizes an array of micro projectors, each projecting a portion of the output sub-image tiled together to create a seamless composite image. Numerous prior art exists that pertains to displays that create an image by tiling its constituent segments. For example, in order to make a cost-effective large screen size LCD displays, U.S. Pat. No. 5,563,470 describes an approach for building large screen LCD displays by tiling smaller size LCD panels. “A Novel Approach to Tiled Displays”, Lowe et al., Society for Information Display (SID) Digest 2000, “Case Study: Building the Market for a Tiled Display Solution”, Needham, Information Display, October 2003, “Seamless Tiling Technology for Large Direct-View Color AMLCD's”, Krusius et al., Society for Information Display (SID) Digest 2000 and “Seamless Tiling of AMLCD's for Large Area Displays”, Krusius et al., Society for Information Display (SID) Digest 2002, all describe a technique for achieving visual continuity across the seams of tiled LCD display panels. U.S. Pat. No. 6,690,337 describes video display system that uses multiple tiled desktop display panels having visible seams with the display panels tilted to achieve visual continuity. Large venue wall displays that use tiling of multiple full size rear projection displays, although with visible seams, are commercially available from several suppliers, for example the stackable display cubes from Samsung and Mitsubishi. U.S. Pat. No. 6,254,239 describes an apparatus for large venue wall displays that use tiling of multiple full size rear projection display systems to generate a composite seamless image. U.S. Pat. Nos. 4,974,073, 5,136,390, 6,115,022 and 6,760,075 describe look-up-table based edge-blending methods for seamless integration of the images generated by multiple tiled video projectors. Although U.S. Pat. Nos. 4,974,073, 5,136,390, 6,115,022, 6,254,239 and 6,760,075 describe methods for seamless tiling of multiple projectors, these methods fail to address the critical issue of maintaining chromatic and luminance uniformity across the multiple projectors used. U.S. Pat. No. 6,568,816 deals with issue of having uniform chromaticity and luminance across the multiple projectors by using a single light source lamp with multiple light beam splitters to distribute the light generated by the single light source lamp to multiple projector heads. In effect the method described in U.S. Pat. No. 6,568,816 would attain a degree of chromatic and luminance uniformity across the multiple projectors at the expense of severely degrading the luminance efficiency due to light lose in the splitters. In general, the apparatus and methods described in U.S. Pat. Nos. 4,974,073, 5,136,390, 6,115,022, 6,254,239, 6,568,816 and 6,760,075 would be suitable for large venue wall displays and their resultant implementation are bulky and as such are not suitable for building consumer desired size and form factor large screen rear projection display devices.
In spite of the numerous existing prior art that pertains to displays that create an image by tiling its segments, none was found that describes an integrated, consumer desired size & form factor large screen (50″ or larger screen diagonal) rear projection display device that generates a seamless composite image that is created by tiling its constituent sub-image segments.
In weighing the pros and cons of the various display technologies, consumers tend to be attracted to the low-profile form factor offered by PDP and LCD displays and often opt for a smaller screen size to overcome the generally higher cost of such displays. Given such a trend and the aforementioned weakness of currently available displays, a rear projection display that offers a high reliability large screen diagonal sizes with a low-profile form factor that is comparable to that of PDP and LCD displays; but with a price point that is comparable to that of current rear projection displays, is certain to have a significant market value.